Extensible link and meta tags in base template

While reviewing old issues today I remembered the work @RawKStar77 was doing on social media integrations.

One part of that was about getting metadata into the standalone page so that annotations had proper stories in Twitter, G+, Facebook, etc.

Here's a simple preparatory step. It allows any of our views to pass a "link" or "meta" array to the renderer. The base template will loop over these arrays expecting dictionary items whose keys and values become the attributes of a link or meta tag.

For instance, we could start to add metadata to our standalone view by adding to `h.views.annotation`:

```python
def annotation(context, request):
    return {
        'meta': [
            {'property': 'og_type', 'content': 'article'},
            {'property': 'og_url', 'content': request.current_resource_url()},
            {'property': 'og_title', 'content': 'Annotation on {}'.format(context.title)},
        ]
    }
```

etc...